Generally, austenitic stainless steel is used in an environment in which corrosion resistance is preferred and possible required, and known examples regulated in JIS are SUS304, SUS316 and SUS317 containing a large amount of Ni and Mo for improvement of corrosion resistance against non-oxygenated acid, and SUS304L, SUS316L, and SUS317L, whose C contents are reduced to improve the grain boundary corrosion resistance, and these types of steels are selected and used according to a corrosive environment.
As welding wires to be used for welding these austenitic stainless steels, the wire for austenitic stainless steel regulated in JIS Z 3321 and the flux-cored wire for austenitic stainless steel regulated in JIS Z 3323 are used in many cases. In addition, flux-cored wires for 308, 316, 308L, and 316L austenitic stainless steels are also used (for example, refer to Japanese Unexamined Patent Application, First Publication No. S58-205696, and Japanese Unexamined Patent Application, First Publication No. S62-68696).
On the other hand, for a hull structure, coated steel plates to which a heavy-duty coating is applied are conventionally used; however, for example, in the usage as a hydrofoil, etc., of a high-speed vessel, high-velocity seawater current comes into contact therewith, so that a high-strength material of an austenitic stainless steel which has excellent seawater corrosion resistance and which does not require coating has been proposed (for example, refer to Japanese Patent Publication No. 2783895 and Japanese Patent Publication No. 2783896).
In addition, for example, highly corrosion-resistant stainless steels such as SUS836L and SUS890L, etc., which contain Cr, Mo, Cu, and N to particularly increase the seawater corrosion resistance and whose pitting corrosion resistance and crevice corrosion resistance are improved by increasing the Mo and N contents compared with those conventionally used, have been developed.
As a welding material to be used when welding these highly corrosion-resistant stainless steels and high seawater corrosion resistance stainless steels, there are proposed a high Mo-high N-based TIG and plasma welding wire for highly corrosion-resistant stainless steel welding, containing Mo: 6.0 through 7.0%, N, 0.25 through 0.50%, Cr: 21.5 through 25.0%, Ni: 17.5 through 20%, and Cu: 0.5 through 1.0% (for example, refer to Japanese Unexamined Patent Application, First Publication No. H01-95895), and a high Mo-high N-based flux-cored wire for highly corrosion-resistant stainless steel welding, containing Mo: 2.7 through 6.7%, N, 0.05 through 0.30%, Cr: 18.6 through 28.9%, Ni: 12.7 through 27.3%, and Cu: 0.8 through 2.4% (for example, refer to Japanese Unexamined Patent Application, First Publication No. H03-86392).
In some cases, a highly corrosion-resistant stainless steel is welded by using a high Cr-high Mo-based Ni alloy wire of Inconel 625 (60Ni-22Cr-9Mo-3.5Nb), etc., without using these common metal welding wires.
When welding a highly corrosion-resistant stainless steel by using the above-described high Mo-high N-based welding wire and high Cr-high Mo-based Ni alloy wire, sufficient seawater corrosion resistance of the weld metal is secured. However, a brittle phase such as a sigma phase is deposited in the weld metal due to a heat cycle caused by welding and significantly lowers the toughness of the weld metal, and in particular, this problem becomes apparent as the Mo content in the wire increases (for example, refer to Onzawa, et. al., Quarterly Journal of the Japan Welding Society Vol. 5 (1987), pp. 262-268).
Generally, in terms of weldability, that is, in terms of prevention of high-temperature solidification cracks of weld metals, components of these welding wires for austenitic stainless steel are designed so that weld metal containing approximately several through ten percent of a ferrite phase in terms of a volume ratio in a welding composition can be obtained by welding. However, weld metal containing a ferrite phase in its weld metal composition becomes lower in low-temperature toughness than a single austenite phase weld metal, and according to an increase in the ferrite amount, this problem becomes apparent (for example, refer to D. T. Read et. al.; Welding Journal, Vol. 59 (1980), pp. 104s-113).
On the other hand, in an austenitic stainless steel welding structure to be applied to a hull structure, etc., that is used in seawater environment and is preferred to be safe in the case of hitting a reef or collision of ships, it is demanded that a weld metal excellent in pitting corrosion resistance in seawater environment, excellent in crevice corrosion resistance, and excellent in low-temperature toughness be provided as a weld zone.